The present invention relates to a semiconductor light-emitting element and its manufacturing method, and a semiconductor light-emitting device and, more particularly, to a semiconductor light-emitting element having a transparent electrode and its manufacturing method.
FIG. 22 shows a longitudinal-sectional structure of a semiconductor light-emitting element to which the present invention is directed. An AuGe n-type electrode (substrate-side electrode; to be referred to as an n-electrode hereinafter) 29 is formed on the first major surface of an n-GaAs substrate 28. Semiconductor layers are stacked in turn on the second major surface of the n-GaAs substrate 28 by, e.g., metal organic chemical vapor deposition (to be referred to as MOCVD hereinafter) or the like. That is, an n-GaAs buffer layer 27 is grown on the second major surface. A reflection layer 26 made up of 10 pairs of n-GaAs/n-In.sub.0.5 Al.sub.0.5 P films is grown on the surface of the buffer layer 27. An n-In.sub.0.5 Al.sub.0.5 P cladding layer (n-cladding layer) 25 is formed on the surface of the reflection layer 26. An In.sub.0.5 (Ga.sub.0.55 Al.sub.0.45).sub.0.5 P active layer 24 is formed on the surface of the n-cladding layer 25. A p-In.sub.0.5 Al.sub.0.5 P cladding layer (p-cladding layer) 23 is formed on the surface of the active layer 24. The p-cladding layer 23, active layer 24, and n-cladding layer 25 make up a light-emitting layer 30 having a doublehetero structure. A current diffusion layer 22 is formed on the light-emitting surface of the light-emitting layer 30, and a p-GaAs ohmic-contact layer 21 is grown on the current diffusion layer 22. An Au p-type electrode (light output electrode; to be referred to as a p-electrode hereinafter) 20 is formed on the ohmic-contact layer 21. These semiconductor layers are formed by, e.g., MOCVD.
A p-type semiconductor layer normally has low resistivity. Hence, when the p-electrode 20 for current injection and active layer 24 are located at neighboring positions, i.e., when the current diffusion layer 22 is not formed, currents spread little, and those portions immediately below the p-electrode 20 and near the electrode readily emit light. As a result, the luminous efficiency lowers considerably.
When currents are concentrated, the forward-bias voltage rises. In order to allow easy spread of currents, a current diffusion layer 22 having relatively low resistance and low absorption with respect to the emission wavelength is often formed. Furthermore, in order to remove light emission immediately below the p-electrode 20, a light-emitting element which has a current blocking layer for blocking currents, between the doublehetero structure and the current diffusion layer, and requires two growth processes, has been proposed. Since the surface of the current blocking layer serves as the surface to be regrown, the current blocking layer is normally formed of a layer containing no Al to avoid problems such as oxidation.
For the current blocking layer containing no Al, GaAs, GaAsP, InGaAs, InGaAsP, and the like may be used. For example, in an InGaAlP-based semiconductor light-emitting element, GaAs is used as a multilayered film formed on the uppermost surface. Since GaAs is not transparent to the emission wavelength in the InGaAlP-based semiconductor light-emitting element, a thin GaAs layer is formed. However, absorption occurs, and light emitted near the peripheral portion below the current blocking layer cannot be effectively output, resulting in low luminous efficiency.
As the current diffusion layer, a GaAlAs layer is used. However, since the GaAlAs layer has a carrier concentration of about 2E18 cm.sup.-3 and a resistivity as high as 7E-2Q cm, spread of currents injected from the p-electrode is insufficient, and light emission cannot spread to the entire surface. For this reason, especially in a green InGaAlP-based semiconductor light-emitting element, the element peripheral portion that does not emit light becomes an absorption band of the emitted light, and light emission is observed at an exit in the lateral direction, i.e., in a direction parallel to the active layer while being shifted to longer wavelength. For this reason, this semiconductor light-emitting element is mounted on a frame without any reflection plate so as not to leak lateral light emission via a lens. hence, light in the lateral direction cannot be effectively used, resulting in low luminous efficiency.
Also, the active layer of a light-emitting diode (LED) using an InGaAlP-based semiconductor to which the present invention is directed is not doped with any impurity intentionally, and as a consequence, the active layer is of n-type having a carrier concentration of about 1E16 cm.sup.-3. Hence, the LED has an n-n.sup.- -p doublehetero structure (see FIG. 22). In this case, as shown in FIG. 22, the minority carriers injected into the active layer are not electrons 31 but holes 32. The luminous efficiency of the LED depends on the minority carrier lifetime. The luminous efficiency of the LED is improved as the minority carriers have less probability of nonradiative recombination in the active layer, i.e., as they have longer lifetime. In this case, since the minority carriers are holes, which normally have a short lifetime, improvement of luminous efficiency is limited.
One of semiconductor light-emitting elements to which the present invention is applied has a transparent electrode and current blocking layer, as shown in FIG. 24. In such light-emitting element, an AuGe n-electrode 202 is formed on the first major surface of an n-GaAs substrate 201, and a plurality of semiconductor layers are stacked on its second major surface by MOCVD.
An n-GaAs buffer layer 203 is formed on the second major surface of the substrate 201, and an n-GaAs/n-In.sub.0.5 Al.sub.0.5 P reflection layer 204, n-In.sub.0.5 Al.sub.0.5 p cladding layer 205, In.sub.0.5 (Ga.sub.0.55 Al.sub.0.45).sub.0.5 P active layer 206, and p-In.sub.0.5 Al.sub.0.5 P cladding layer 207 are formed in turn on the buffer layer. The n-cladding layer 205, active layer 206, and p-cladding layer 207 form a doublehetero layer 200.
A p-Ga.sub.0.2 Al.sub.0.8 As current diffusion layer 208 and p-GaAs ohmic-contact layer 209 are formed in turn on the top surface of the doublehetero layer 200, and an AuZn p-electrode 210 is formed on the surface of the contact layer 209.
As described above, the element to which the present invention is directed uses a GaAlAs-based semiconductor as the current diffusion layer 208. However, the resistivity of this layer 208 is too high to obtain sufficient spread of currents. For this reason, currents concentrate on the p-electrode 210. As a result, the internal quantum efficiency lowers due to heat generation, or the p-electrode 210 absorbs light, resulting in low light output efficiency.
As described above, the light-emitting element shown in FIG. 22 has low luminous efficiency. On the other hand, the light-emitting element shown in FIG. 24 has low internal quantum efficiency and light output efficiency.